1. Field of the Invention
The present invention relates to a rotary feed mechanism which rotates and feeds one of a pair of annular members, wherein the annular members engage with each other through a multiple thread (helicoid), through a gear arrangement.
2. Description of Related Art
For instance, in a known lens barrel of a camera, a movable (inner) ring which is screw-engaged by a stationary (outer) ring, through a multiple thread (helicoid), is moved (fed) in an optical axis direction when the movable ring is rotated with respect to the stationary ring. In particular, in a known zoom compact camera, the movable ring is provided, on the outer peripheral surface thereof, with a gear which is in mesh with a pinion driven by a motor to drive the movable ring.
In such a known rotary feed mechanism, it is necessary to provide both the multiple thread and the gear on the outer peripheral surface of the movable ring. To shorten the axial length of the movable ring, it is known to cut away a part of the multiple thread, so that the gear can be provided in the cut-away portion in such a way that the gear is inclined in the same direction as that of the multiple thread. Namely, the provision of the gear in the cut-away portion of the multiple thread makes it possible to shorten the axial length of the movable ring as a whole, in comparison with an arrangement in which the gear and the multiple thread are located at different axial positions.
To increase the axial displacement (feed) of the movable ring, it is necessary to increase the number of gear teeth or lengthen the pinion and widen the gear so that the pinion and gear engagement has an increased axial length. To increase the number of gear teeth, however, leads to an increase in the axial length of the movable ring since the gear extends along the multiple thread in a direction inclined with respect to the axis of the movable ring. On the other hand, if the gear width is increased, the circumferential area of the movable ring in which the multiple thread can be formed is accordingly decreased, resulting in an insufficient degree of engagement with the stationary ring. This may cause the movable ring to be askew with respect to the stationary ring.
In order to increase a zoom ratio of a zoom lens in a camera which is to be made as compact as possible, it is necessary to provide a sufficient amount of feed of the movable ring without making the latter askew with respect to the stationary ring. No conventional feed mechanism satisfactorily accomplishes this requirement.